I. Field
The present invention relates generally to data communication, and more specifically to orthogonal frequency division multiplexing (OFDM) communication systems and techniques for providing OFDM symbol sizes to increase wireless efficiency.
II. Background
Wireless communication systems are widely deployed to provide various types of communication services such as voice, packet data, and so on. These systems may utilize OFDM, which is a modulation technique capable of providing high performance for some wireless environments. OFDM effectively partitions the overall system bandwidth into a number of (NS) orthogonal subbands, which are also commonly referred to as tones, bins, and frequency subchannels. With OFDM, each subband is associated with a respective carrier that may be modulated with data.
In OFDM, a stream of information bits is converted to a series of frequency-domain modulation symbols. One modulation symbol may be transmitted on each of the NS subbands in each OFDM symbol period (defined below). The modulation symbols to be transmitted on the NS subbands in each OFDM symbol period are transformed to the time-domain using an inverse fast Fourier transform (IFFT) to obtain a “transformed” symbol that contains NS samples. The input to an NS-point IFFT is NS frequency-domain values and the output from the IFFT is NS time-domain samples. The number of subbands is determined by the size of the IFFT. Increasing the size of the IFFT increases the number of subbands and also increases the number of samples for each transformed symbol, which correspondingly increases the time required to transmit the symbol.
To combat frequency selective fading in the wireless channel used for data transmission (described below), a portion of each transformed symbol is typically repeated prior to transmission. The repeated portion is often referred to as a cyclic prefix, and has a length of NCP, samples. The length of the cyclic prefix is typically selected based on the delay spread of the system, as described below, and is independent of the length of the transformed symbol. An OFDM symbol is composed of a transformed symbol and its cyclic prefix. Each OFDM symbol contains NS+Ncp samples and has a duration of NS+Ncp sample periods, which is one OFDM symbol period.
The size of the cyclic prefix relative to that of the OFDM symbol may have a large impact on the efficiency of an OFDM system. The cyclic prefix must be transmitted with each OFDM symbol to simplify the receiver processing in a multipath environment but carries no additional information. The cyclic prefix may be viewed as bandwidth that must be wasted as a price of operating in the multipath environment. The proportion of bandwidth wasted in this way can be computed using the formula
            N      cp                      N        S            +              N        cp              .For example, if Ncp is 16 samples and NS is 64 samples, then 20% of the bandwidth is lost to cyclic prefix overhead. This percentage may be decreased by using a relatively large value of NS. Unfortunately, using a large value of NS can also lead to inefficiency, especially where the size of the information unit or packet to be transmitted is much smaller than the capacity of the OFDM symbol. For example, if each OFDM symbol can carry 480 information bits, but the most common packet contains 96 bits, then packing efficiency will be poor and much of the capacity of the OFDM symbol will be wasted when this common packet is sent.
Orthogonal frequency division multiple-access (OFDMA) can ameliorate the inefficiency due to excess capacity resulting from the use of a large OFDM symbol. For OFDMA, multiple users share the large OFDM symbol using frequency domain multiplexing. This is achieved by reserving a set of subbands for signaling and allocating different disjoint sets of subbands to different users. However, data transmission using OFDMA may be complicated by various factors such as, for example, different power requirements, propagation delays, Doppler frequency shifts, and/or timing for different users sharing the large OFDM symbol.
Existing OFDM systems typically select a single OFDM symbol size that is a compromise of various objectives, which may include minimizing cyclic prefix overhead and maximizing packing efficiency. The use of this single OFDM symbol size results in inefficiency due to excess capacity when transmitting packets of varying sizes. There is therefore a need in the art for an OFDM system that operates efficiently when transmitting packets of varying sizes.